*3.1. Principle of Electricity Measurement*

Most industrial customers equipped with three-phase smart meters. According to the different wiring modes, it can be divided into two types: three-phase four-wire and three-phase three-wire. In this paper, we primary introduce our approach with three-phase four-wire as an example. For single phase scenario, the electricity energy is calculated by following equation:

$$E = \sum\_{n=0}^{N-1} P\_n \cdot \Delta t \tag{1}$$

where, Δ*t* is the cycle of computation, *E* is the active electricity energy in a certain time period *N* · Δ*t*, *Pn* is the average active power at time n. It can be further calculated by voltage and current:

$$P\_n = \mathcal{U}\_n \cdot I\_n \cdot \cos \phi\_n \tag{2}$$

where, *Un* and *In* are average voltage and current at time *n*, *φn* is the phase difference between voltage and current at the same time stamp and *cosφn* is called power factor. Further, the equation of active energy are rewritten by:

$$E = \sum\_{n=0}^{N-1} \left[ \mathcal{U}\_n \cdot I\_n \cdot \cos \phi\_n \right] \cdot \Delta t \tag{3}$$

For three-phase four-wire situation, the total active electricity energy is calculated by:

$$E\_{total} = \sum\_{n=0}^{N-1} P\_{total} \cdot \Delta t = \sum\_{n=0}^{N-1} \left[ P\_A^\text{\textquotedbl} + P\_B^\text{\textquotedbl} + P\_C^\text{\textquotedbl} \right] \cdot \Delta t \tag{4}$$

Generally, electricity energy measured by voltage and current, and the relationship between them is very important to NTL detection.
